Add support for Build contextDir in Buildpacks

[qu1queee]

Idea:
Currently specifying a subpath where the source code is located does not work for Buildpacks strategies. This is done via the contextDir key. For Buildpacks, it would be interesting to have sample files for different runtimes. For example this repository host many different runtime applications, so using a single repo source for future buildpacks samples would be ideal. But without the contextDir support, this is not possible.

Acceptance_Criteria:
Add the missing functionality in buildpacks strategies, PR this and ensure all tests are happy.

[sbose78]

Got it, so the buildpacks strategy doesn't acknowledge contextDir at the moment - and that's gotta be fixed.